Tooling and methods for circumferential friction stir welding

ABSTRACT

A back-up tooling apparatus for supporting the inner side of a hollow cylindrical workpiece during a circumferential friction stir weld includes a plurality of arcuate shoes configured to mate end-to-end to form a substantially continuous ring, and an expandable support for supporting the shoes and operable to retract and expand the shoes inward and outward. The expandable support includes at least one rotatable feed screw and central support members mounted on the feed screw or screws and pivotally connected to the shoes by support arms. The central support members are moved axially toward or away from each other for expanding or retracting the shoes. The shoes in one embodiment overlap in the circumferential direction, and a seal material is disposed between mating end faces of adjacent shoes. A restraining device is provided for clamping about the workpiece to reduce thermal expansion thereof. Cable-and-winch and geared drive devices are provided for rotatably driving the workpiece from its outer surface.

FIELD OF THE INVENTION

The invention relates to friction stir welding and, more particularly,to tooling and methods for making circumferential friction stir welds oncylindrical workpieces such as tanks, rocket casings, and the like.

BACKGROUND OF THE INVENTION

Friction stir welding is a relatively new technology that has beendeveloped for welding aluminum alloys and other materials. The frictionstir welding process generally involves engaging the material of twoadjoining workpieces on either side of a joint by a rotating stir pin orspindle. Force is exerted to urge the spindle and the workpiecestogether, and frictional heating caused by the interaction between thespindle and the workpieces results in plasticization of the material oneither side of the joint. The spindle is traversed along the joint,plasticizing material as it advances, and the plasticized material leftin the wake of the advancing spindle cools and solidifies to form aweld.

It will be appreciated that large forces must be exerted between thespindle and the workpieces in order to apply sufficient pressure to theworkpieces to cause plasticization of the material. For instance, forfriction stir welding an aluminum alloy plate of ¼-inch thickness,forces of up to 4000 pounds or more may have to be exerted between thespindle and the plate. In a conventional friction stir welding process,these large forces are absorbed at least partially by a back-up memberwhich engages the workpieces on the “back side” of the weld oppositefrom the spindle. Where the workpieces have sufficient structuralstrength and rigidity, part of the welding forces may be absorbed by theworkpieces themselves. However, in many cases the workpieces aresemi-flexible structures which are incapable of supporting and absorbingthe large forces involved in a friction stir welding process.Accordingly, the back-up member is usually supported by a substantialsupport structure.

A number of challenges are presented in friction stir welding a hollowcylindrical workpiece. Because of limited space inside the workpiece,the rotating friction stir welding tool generally must engage theworkpiece from the outside and a suitable back-up tool must support theinner surface of the workpiece along its entire circumference tocounteract the large inward forces exerted on the workpiece by thewelding tool. A one-piece or fixed geometry back-up tool is impractical,and could not be used in workpieces in which the opening in theworkpiece through which the back-up tool must be inserted is smaller indiameter than the portion of the workpiece to be welded. Thus, theback-up tool must be constructed from a plurality of members that can beplaced inside the workpiece and then assembled into a full-circumferenceback-up tool.

The multi-component construction of the back-up tool is not optimum fromthe standpoint of rigidity of the tool. Tool rigidity is importantbecause, unless the back-up tool has sufficient rigidity, the weldingforces can cause deformations of the workpiece, leading to problems suchas irregular welds. Accordingly, one challenge in friction stir weldinghollow cylindrical workpieces is providing a back-up tool that can fitthrough an opening in the workpiece and can be assembled into afull-circumference back-up tool having sufficient rigidity to preventexcessive deformation of the workpiece during welding. The back-up toolshould also be capable of being assembled and disassembled relativelyquickly.

Another problem encountered in friction stir welding a cylindricalstructure along a circumferential weld path is that the heat generatedduring the welding process tends to cause radial growth of the structurethrough thermal expansion. As a result, the welding tool tends to becomeburied in the weld metal, causing excessive metal flash and voiding.

Still another problem in friction stir welding a cylindrical structurerelates to the rotational driving of the structure. The friction stirwelding tool remains in one place and the cylindrical workpiece isrotatably driven about its axis to cause the welding tool to traverse acircumference of the workpiece. Prototype welding equipment developed bythe assignee of the present application employed a rotary drivemechanism that drove the workpiece by means of an arm that engaged theworkpiece and rotated about an axis coinciding with the axis of theworkpiece. Thus, the torque arm of the drive mechanism was essentiallyequal to the radius of the workpiece. It will be appreciated that forlarge-diameter workpieces, the resulting torque requirement for thedrive mechanism could be quite large. For instance, assuming ahorizontal welding load of 2000 pounds that must be overcome by thedrive mechanism, a 16-foot diameter workpiece would require a drivetorque of 16,000 foot-pounds.

A further drawback of the prototype center-drive mechanism is that thedrive arm tended to flex, which caused imprecise control of therotational motion of the workpiece. For instance, at the end of a weldwhen the drive mechanism was stopped and the weld tool was withdrawnfrom the workpiece, the return of the drive arm to a relaxed conditionresulted in some further rotational movement of the workpiece, causingan elongation in the exit “keyhole” formed by the withdrawal of the weldtool. Additionally, when starting the drive mechanism to begin a weldingoperation, the flexing of the drive arm resulted in some backlash suchthat movement of the workpiece did not begin precisely when commandedand the speed of the workpiece was not as uniform as desired. Theseproblems were noted in welding 3-foot diameter tanks. Withlarger-diameter structures, the problems caused by drive arm flexurelikely would be even greater.

SUMMARY OF THE INVENTION

The present invention provides tooling and methods that, at least inpreferred embodiments, overcome some or all of the problems noted above.In accordance with a first aspect of the invention, a back-up toolingapparatus is provided for backing up an inner surface of a hollowcylindrical workpiece to be friction stir welded along a circumferentialweld path. The back-up tooling apparatus comprises a plurality of shoeseach having an arcuate shape in a circumferential direction of theworkpiece, the shoes being configured to mate end-to-end with oneanother so as to form a substantially continuous ring having an outerdiameter approximately equal to a diameter of the inner surface of theworkpiece. The shoes are supported by a radially expandable supporthaving a plurality of support arms respectively attached to theplurality of shoes. The expandable support is operable to retract thesupport arms and shoes inwardly to permit the back-up tooling apparatusto be inserted inside the workpiece, and is operable to extend thesupport arms and shoes outwardly to cause the shoes to mate with oneanother in end-to-end relation to form the continuous ring and to urgethe ring against the inner surface of the workpiece. To enable the shoesto be retracted inwardly, alternate shoes in the circumferentialdirection preferably are axially staggered with respect to the remainingshoes when the expandable support is retracted.

In accordance with a preferred embodiment of the invention, theexpandable support comprises at least first and second axially spacedcentral support members that are relatively movable toward and away fromeach other along an axis of the expandable support, and each shoe hassupport arms that are pivotally attached to the shoe and extend fromopposite sides of the shoe to the central support members. Relativemovement of the central support members toward each other causes theshoe to be moved radially outward and movement of the central supportmembers away from each other causes the shoe to be moved radiallyinward.

The expandable support preferably comprises a screw-feed device havingthe first central support member threadingly mounted on a rotatable feedscrew, the feed screw being rotated in one direction or another to causethe first central support member to move relatively toward or away fromthe second central support member. To provide the axial staggering ofsome shoes relative to other shoes when the shoes are retracted, theback-up tooling apparatus preferably includes a third central supportmember threadingly mounted on the rotatable feed screw axially spacedfrom the first central support member. Alternate shoes in thecircumferential direction are connected via support arms to the firstcentral support member and the remaining shoes are connected via supportarms to the third central support member. Thus, when the first and thirdcentral support members are moved away from the second central supportmember to retract the shoes, the shoes connected to the first and thirdcentral support members are moved different distances in the axialdirection.

To facilitate mating of the shoes when they are expanded outward, themating end faces of circumferentially adjacent shoes advantageously arecomplementarily angled relative to the axial direction.

It is further advantageous to configure the shoes so that they overlapin the circumferential direction when the shoes are expanded to form thering such that weld pressure exerted radially inward on the overlappingend portions causes the overlapping end portions to be forced together.The joints between adjacent shoes thus are “self-sealing”, which helpsprevent the creation of excessively large gaps between shoes. Such gapscould result in plasticized weld metal being forced through the gaps. Tofurther improve the connection between shoes, a seal materialadvantageously is disposed between mating end faces of the overlappingend portions of the shoes. The seal material can be any material that issofter than the shoes and will not melt or be forced out from betweenshoes under the temperatures and pressures expected to be imposed on thematerial during use. Examples of suitable seal material include copperand polytetrafluoroethylene.

The support and rigidity provided by the back-up tooling apparatus canbe enhanced by careful construction of the shoes. More particularly, apreferred embodiment of the invention provides an apparatus wherein someof the shoes are configured to become circumferentially narrower in aradially outward direction, and the other shoes are configured to becomecircumferentially wider in the radially outward direction. The narrowingshoes are alternately disposed with respect to the widening shoes aboutthe circumference so as to form an interlocking arrangement. The effectof this arrangement is that force exerted radially outward on anarrowing shoe is transmitted to adjacent widening shoes. In otherwords, the narrowing shoes tend to support the widening shoes, and theentire ring is thus made more rigid.

The invention in other embodiments also helps alleviate the problem ofthermal growth of the workpiece. To this end, the back-up toolingapparatus preferably includes an outer restraining device configured toencircle an outer surface of the workpiece and operable to resistoutward expansion of the workpiece. The restraining deviceadvantageously comprises a pair of ring-shaped restraining bars adaptedto be mounted about the workpiece on opposite sides of thecircumferential weld path. Each restraining bar conveniently is formedin two or more arcuate segments that are fastened together to clampabout the workpiece.

In still other embodiments, the invention also helps alleviate theproblem of inaccurate driving of the workpiece caused by flexure in thedrive system. Thus, in accordance with a preferred embodiment of theinvention, the back-up tooling apparatus includes a drive device inengagement with an outer surface of the workpiece and operable to rotatethe workpiece about its axis. The drive device preferably comprises acable attached to and wrapped about the workpiece and a winch connectedto the cable for pulling the cable to rotate the workpiece. Preferably,the cable is attached to the restraining device that encircles theworkpiece. Alternatively, a member having an outer geared surface isaffixed about the workpiece and the drive device comprises at least onerotatably driven gear meshed with the geared surface for rotating theworkpiece. Thus, the torque required to drive the workpiece isessentially independent of the diameter of the workpiece. Flexure in thedrive device can be reduced or substantially eliminated so thatmore-precise control of the workpiece movement can be attained.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the inventionwill become more apparent from the following description of certainpreferred embodiments thereof, when taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a side elevation of a back-up tooling apparatus in accordancewith a preferred embodiment of the invention, shown in a retractedcondition inside a cylindrical workpiece to be welded;

FIG. 2 is a side elevation similar to FIG. 1, but with the apparatus inan expanded condition engaging the inner surface of the workpiece;

FIG. 2A is an enlarged view of a portion of FIG. 2, showing the featureof angled end faces of the shoes to facilitate mating of the shoes;

FIG. 3 is an end elevation of one of the shoes of the apparatus in FIG.1;

FIG. 4 is an end elevation of another embodiment of the invention,showing a plurality of shoes expanded out to form a substantiallycontinuous ring;

FIG. 5 is a fragmentary end elevation of a ring in accordance with afurther embodiment of the invention in which a seal material is disposedbetween mating end faces of the shoes and the shoes are configured suchthat weld pressure tends to increase the sealing between shoes;

FIG. 6 is an end elevation of a restraining device for reducing thermalgrowth of a workpiece during a welding operation;

FIG. 7 is an end elevation showing a workpiece and a cable device forrotatably driving the workpiece;

FIG. 8 is an enlarged fragmentary end elevation showing a cable attachedto a restraining device that is clamped about a workpiece; and

FIG. 9 is an end elevation showing a workpiece and a geared drive devicefor rotatably driving the workpiece.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. Likenumbers refer to like elements throughout.

With reference to FIGS. 1-3, a back-up tooling apparatus 20 inaccordance with a preferred embodiment of the invention is shown. InFIGS. 1-2, the apparatus is shown inside a cylindrical workpiece W to bewelded along a circumferential weld path by a friction stir welding toolT (FIG. 2) that is disposed outside the workpiece W. The friction stirwelding process itself is known and thus will not be described furtherherein. The process is described in general in U.S. Pat. No. 5,460,317,the entire disclosure of which is incorporated herein by reference.Suffice it to say here that the welding tool T is rotated about its axisand is plunged in a direction generally along its axis into the materialof the workpiece to be welded. Friction between the tool and thematerial causes the material to become plastic. The welding tool andworkpiece are moved relative to each other so that the welding tooltraverses a path through the workpiece. A shoulder on the tool engagesthe outer surface of the workpiece, and as the tool is rotated andadvanced through the workpiece, axial force on the tool causes theshoulder to frictionally engage and plasticize the material. In theinstant case, the welding tool T preferably is held in one place and theworkpiece W is rotated about its axis to cause the tool T to traverse acircumferential welding path. Although not shown in the drawings, itwill be understood that the workpiece W being welded actually consistsof two parts having cylindrical portions that are abutted against eachother end-to-end to form a circumferential joint, and this joint definesthe welding path that is traversed by the welding tool T.

In a friction stir welding process, some sort of backing member isrequired on the back side of the workpiece in order to contain thewelding pressure so that plasticized weld metal is not forced throughthe back side of the workpiece. The backing member in many cases alsosupports the workpiece against the large forces that are imposed on itby the welding tool.

The axial force on the welding tool T can be quite large, for example2000-4000 pounds or more, depending on the material and thickness of theworkpiece and other factors. As the welding tool progresses through theworkpiece along the welding path, a large sideward force is also exertedon the workpiece. These welding forces are transmitted to and absorbedby the backing member. The backing member preferably should besufficiently rigid to prevent excessive deformations of the backingmember and workpiece.

In accordance with the present invention, the back-up tooling apparatus20 is provided for stabilizing the workpiece W and for containing theweld pressure. The apparatus 20 includes a plurality of arcuate shoes 22and 24 that can be assembled end-to-end to form a substantiallycontinuous ring. The apparatus 20 also includes an expandable support 26that supports the shoes 22, 24 and is operable to retract the shoesinward as shown in FIG. 1 to reduce the overall diameter of theapparatus so that it can be inserted through an opening in the end ofthe workpiece (which opening may be smaller in diameter than the portionof the workpiece to be welded), and to expand the shoes outward once theapparatus is inside the workpiece, as shown in FIG. 2, so that the shoesmate with one another to form a ring that engages the inner surface ofthe workpiece.

The expandable support 26 in the preferred embodiment illustratedcomprises a rotatable feed screw 28, a plurality of axially spacedcentral support members 30, 32, and 34 at least two of which (i.e., themembers 32 and 34) are threadingly mounted on the feed screw 28, and aplurality of support arms 36 pivotally connected between the shoes 22,24 (see FIG. 3) and the central support members 30, 32, and 34. Thecentral support members 32 and 34 are prevented from rotating along withthe feed screw 28 by suitable means (not shown) so that rotation of thefeed screw causes the support members 32 and 34 to translate along thefeed screw toward the remaining central support member 30. The centralsupport member 30 can also be threadingly mounted on the feed screw 28on a portion thereof that is threaded with a reverse pitch relative tothe portion on which the support members 32 and 34 are mounted, suchthat rotation of the feed screw 28 causes the support member 30 to alsotranslate toward the other support members 32 and 34. Alternatively, theapparatus 20 can employ more than one independently operable feed screw,with the support members 30-34 mounted on different feed screws so thatmovement of one of the support members can be controlled independentlyof movement of the other support members.

Each of the shoes 22 is connected by a support arm 36 to the supportmember 30 and by another support arm 36 to the support member 32.Similarly, each of the shoes 24 is connected by a support arm 36 to thesupport member 30 and by another support arm 36 to the support member34. The shoes 22 preferably are alternately disposed with the shoes 24about the circumference of the apparatus 20. Thus, translation of thesupport members 32 and 34 relatively toward the other support member 30causes the shoes 22 and 24 to be moved in a radial direction.Furthermore, because the support members 32 and 34 are spaced differentaxial distances from the other support member 30, the shoes 22 undergorelative axial movement with respect to the shoes 24 when the feed screw28 is rotated to cause translation of the support members. This can beseen by comparing FIG. 1 with FIG. 2. The support members 30-34 andsupport arms 36 are suitably configured such that the shoes 22, 24 canbe moved into end-to-end mating arrangement to form a complete ring withthe ends of adjacent shoes in contact with each other.

The shoes 22, 24 preferably are configured so that when they cometogether to form a complete ring as shown in FIG. 2, the outer diameterof the ring is at least as large as the relaxed inner diameter of theworkpiece W. More preferably, the shoes 22, 24 can be configured suchthat the ring is slightly larger in diameter than the relaxed innerdiameter of the workpiece, with the result that expansion of theapparatus 20 against the inner surface of the workpiece applies apre-load to the workpiece. This can be advantageous, for example, inorder to counteract thermal expansion of the workpiece as a result offrictional heating during a welding operation, so that the workpiecedoes not expand and lift away from the shoes.

To facilitate mating of the shoes 22, 24, it may be advantageous toprovide end faces 38 that are angled relative to the axial direction asshown in FIG. 2A. This configuration reduces the likelihood ofinterference between the shoes as they are moved axially and radiallytogether.

Various numbers and configurations of shoes can be used in accordancewith the present invention. Certain shoe configurations, however, may bemore advantageous than others. For example, the present invention inpreferred embodiments provides shoe configurations that facilitatesupport of a given shoe by other shoes in the ring. With reference toFIG. 4, a ring is shown made up of shoes 40 alternately disposed withshoes 42 about the circumference. The shoes 40 are configured such thattheir circumferential width increases in the radially outward direction.Conversely, the shoes 42 are configured such that their circumferentialwidth decreases in the radially outward direction. It will beappreciated, therefore, that a radially inward force imposed on awidening shoe 40 will be transmitted to adjacent narrowing shoes 42. Thenarrowing shoes 42 thus help support the widening shoes 40 against theforces imposed by the welding tool, thereby reducing the tendency of agiven shoe to “sink” under load. The spreading of load from one shoe toother shoes in the ring tends to improve the rigidity and stability ofthe ring.

A further advantageous shoe configuration is shown in FIG. 5. A portionof a ring is shown made up of shoes 44, 46, and 48. The end portions ofthe shoes overlap in the circumferential weld direction, so that, forexample, the end portion of the shoe 46 that overlaps the end portion ofthe shoe 48 has a mating face 50 that faces inward. The end portion ofthe shoe 48 has a complementary mating face 52 that faces outward andmates with the face 50. Accordingly, when weld pressure is exerted onthe end portion of the shoe 46, the respective mating faces 50 and 52are forced together, thereby tending to reduce or eliminate any gaptherebetween. In a further preferred embodiment of the invention, asealing material 54 is interposed between the mating faces 50 and 52 tofurther improve sealing between the shoes. The sealing material 54 canbe any material that is softer than the shoes and will not melt or beforced out from between shoes under the temperatures and pressuresexpected to be imposed on the material during use. Examples of suitablesealing materials include copper and polytetrafluoroethylene.

With reference to FIGS. 2 and 6, a further aspect of the invention isthe provision of a restraining device for preventing or reducing thermalgrowth of a workpiece so as to maintain proper contact between theworkpiece and the shoes of the back-up tooling apparatus. Therestraining device is substantially rigid and encircles the outersurface of the workpiece. Preferably, the restraining device comprises apair of ring-shaped restraining bars 56 that can be clamped about theworkpiece W on opposite sides of the weld path. Each restraining bar 56preferably is formed in two or more arcuate segments that are fastenedtogether, such as by bolts 58 (FIG. 6), to form a full ring.Alternatively or in addition to using the restraining device, theworkpiece can be cooled by circulating a gaseous coolant through theinterior of the workpiece during a welding operation in order to reducethermal expansion of the workpiece.

The invention also provides improved devices and methods for rotatablydriving a workpiece to effect a circumferential friction stir weld. FIG.7 shows a cable-drive device for rotating a workpiece. The cable-drivedevice includes a rotatable winch 60 operable to wind a cable 62 ontothe winch. The cable 62 is wrapped about the workpiece W and affixedthereto by a suitable fastening arrangement. As shown in FIG. 8, onesuch fastening arrangement is the restraining bar 56 described above,which can be constructed to securely clamp about the workpiece withsufficient clamping force that the restraining bar will not sliprelative to the workpiece. The cable 62 is affixed to the restrainingbar 56 (or to another fastening arrangement) at one end, and the otherend of the cable is wound about the winch 60. The winch 60 is operatedto wind the cable 62 onto the winch so as to rotatably drive theworkpiece.

An alternative drive device in accordance with the invention is shown inFIG. 9. In this embodiment, a member 64 having a geared outer surface isaffixed about the workpiece W, and at least one rotatable drive gear 66meshes with the member 64 and is rotatably driven to rotate theworkpiece. The gear teeth on the geared outer surface of the member 64and on the drive gear 66 are not shown in FIG. 9

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

What is claimed is:
 1. A back-up tooling apparatus for backing up aninner surface of a hollow cylindrical workpiece to be friction stirwelded along a circumferential weld path, comprising: a plurality ofshoes each having an outer surface that is arcuate in a circumferentialdirection of the workpiece, the shoes being configured to mateend-to-end with one another so as to form a substantially continuousring having an outer diameter approximately equal to a diameter of theinner surface of the workpiece; and a radially expandable support havinga plurality of support arms respectively attached to the plurality ofshoes, the expandable support being operable to extend the support armsand shoes outwardly to cause the shoes to mate with one another inend-to-end relation to form said continuous ring and to urge the shoesagainst the inner surface of the workpiece, wherein the expandablesupport is operable to retract the shoes inward to reduce the diameterof the apparatus, and wherein alternate shoes in the circumferentialdirection are axially staggered with respect to the remaining shoes whenthe expandable support is retracted.
 2. The back-up tooling apparatus ofclaim 1, wherein the expandable support comprises at least first andsecond central support members that are relatively movable toward andaway from each other along an axis of the expandable support, andwherein each shoe has support arms that are pivotally attached to theshoe and to the central support members such that relative movement ofthe central support members toward each other causes the shoe to bemoved radially outward and movement of the central support members awayfrom each other causes the shoe to be moved radially inward.
 3. Theback-up tooling apparatus of claim 1, wherein the expandable supportcomprises a screw-feed device having at least the first central supportmember threadingly mounted on a rotatable feed screw, the feed screwbeing rotated in one direction or another to cause the first centralsupport member to move relatively toward or away from the second centralsupport member.
 4. The back-up tooling apparatus of claim 3, furthercomprising a third central support member axially spaced from the firstcentral support member and axially movable with respect to the secondcentral support member, alternate shoes in the circumferential directionbeing connected via support arms to the first central support member andthe remaining shoes being connected via support arms to the thirdcentral support member, the expandable support being configured to causeaxial staggering of said alternate shoes with respect to said remainingshoes when the first and third central support members are moved awayfrom the second central support member to retract the shoes.
 5. Theback-up tooling apparatus of claim 4, wherein the central supportmembers are mounted on a common feed screw.
 6. The back-up toolingapparatus of claim 2, wherein mating end faces of circumferentiallyadjacent shoes are angled relative to the axial direction.
 7. A back-uptooling apparatus for backing up an inner surface of a hollowcylindrical workpiece to be friction stir welded along a circumferentialweld path, comprising: a plurality of shoes each having an outer surfacethat is arcuate in a circumferential direction of the workpiece, theshoes being configured to mate end-to-end with one another so as to forma substantially continuous ring having an outer diameter approximatelyequal to a diameter of the inner surface of the workpiece; and aradially expandable support having a plurality of support armsrespectively attached to the plurality of shoes, the expandable supportbeing operable to extend the support arms and shoes outwardly to causethe shoes to mate with one another in end-to-end relation to form saidcontinuous ring and to urge the shoes against the inner surface of theworkpiece, wherein mating end portions of circumferentially adjacentshoes are configured to overlap in the circumferential direction whenthe shoes are expanded to form the ring such that weld pressure exertedradially inward on the overlapping end portions causes the overlappingend portions to be forced together.
 8. The back-up tooling apparatus ofclaim 7, further comprising a seal material disposed between mating endfaces of the overlapping end portions of the shoes.
 9. A back-up toolingapparatus for backing up an inner surface of a hollow cylindricalworkpiece to be friction stir welded along a circumferential weld path,comprising: a plurality of shoes each having an outer surface that isarcuate in a circumferential direction of the workpiece, the shoes beingconfigured to mate end-to-end with one another so as to form asubstantially continuous ring having an outer diameter approximatelyequal to a diameter of the inner surface of the workpiece; a radiallyexpandable support having a plurality of support arms respectivelyattached to the plurality of shoes, the expandable support beingoperable to extend the support arms and shoes outwardly to cause theshoes to mate with one another in end-to-end relation to form saidcontinuous ring and to urge the shoes against the inner surface of theworkpiece; and a restraining device configured to encircle an outersurface of the workpiece and operable to resist outward expansion of theworkpiece.
 10. The back-up tooling apparatus of claim 9, wherein therestraining device comprises a pair of ring-shaped restraining barsadapted to be mounted about the workpiece on opposite sides of thecircumferential weld path.
 11. The back-up tooling apparatus of claim10, wherein the restraining bars are formed of a heat-conductingmaterial so as to operate as a heat sink.
 12. The back-up toolingapparatus of claim 10, further comprising a drive device in engagementwith an outer surface of the workpiece and operable to rotate theworkpiece about its axis.
 13. The back-up tooling apparatus of claim 12,wherein the drive device comprises a cable attached to and wrapped aboutthe workpiece and a winch connected to the cable for pulling the cableto rotate the workpiece.
 14. The back-up tooling apparatus of claim 13,further comprising a member having an outer geared surface, the memberbeing affixed about the workpiece, and wherein the drive devicecomprises at least one rotatably driven gear meshed with the gearedsurface for rotating the workpiece.
 15. A back-up tooling apparatus forbacking up an inner surface of a hollow cylindrical workpiece to befriction stir welded along a circumferential weld path, comprising: aplurality of shoes each having an outer surface that is arcuate in acircumferential direction of the workpiece, the shoes being configuredto mate end-to-end with one another so as to form a substantiallycontinuous ring having an outer diameter approximately equal to adiameter of the inner surface of the workpiece, each shoe being incontact with shoes on either side thereof; the shoes being alternatelyconfigured to become circumferentially narrower and wider in a radiallyoutward direction to form an interlocking arrangement such that forceexerted radially outward on a narrowing shoe is transmitted to adjacentwidening shoes, and wherein inward-facing end faces of the wideningshoes overlap outward-facing end faces of adjacent narrowing shoes, andfurther comprising a seal material disposed between the overlapping endfaces.
 16. The back-up tooling apparatus of claim 15, wherein the sealmaterial comprises a strip of material having a lower hardness than theshoes.